For many applications, in which linear movements must ultimately be made, a linear motor as a direct drive may provide advantages over a conventional rotary drive. Since no mechanical elements, such as ball spindles or toothed belts, are required for converting a rotational movement into a linear movement, the possible conveying speed and the positioning accuracy of such a direct drive may not be unnecessarily limited. In this context, speed and force may be controlled over a wide range. Problems with reversing play do not occur during the positioning. In the case of linear direct drives, a comparatively low amount of wear occurs with rapid and frequent changes of direction, so that the service life increases in comparison with rotary drives, and, in the process, the positioning accuracy may not decrease with the operating time.
These features make linear motors interesting for, e.g., pick-and-place applications. In this case, e.g., individual electronic circuits (chips) of a diced silicon wafer must be received and inserted into a housing. Since the chips are very small, several tens of thousands of chips may fit on a wafer of normal size. On one hand, a gripping arm must be able to work very rapidly (several chips per second), and, on the other hand, it must be positioned in a very exact manner, in order to avoid damaging the chips while receiving them, and to be able to position them with the accuracy necessary for further processing.
Waste heat, which causes the operating temperature to increase, is formed in the coils of the primary part of an electric motor. Since temperatures that are too high may lead to the destruction of the motor, one may not often dispense with a cooling system.
In European Patent No. 0 793 870, a synchronous motor (linear or rotary) having a cooling system is described, whose primary part also contains cooling tubes in the gaps of an iron ore, next to the coils. Coolant flows through the cooling tubes, in order to remove heat from the motor. Primary parts of this kind may be encapsulated with a synthetic resin, in order to ensure that the coils, the leads for the coils, and the cooling tubes are securely supported. The use of cooling coils means an additional expenditure and consequently increases the cost of a linear motor to a considerable extent.
In, for example, FIG. 8 of U.S. Pat. No. 4,839,545, is shown an iron core, whose plates have separate cooling channels. However, these cooling channels can be blocked by synthetic resin, when they are situated close to the coils (in order to ensure effective cooling). In order to completely encapsulate the coils, synthetic resin must initially be introduced beyond the fill level actually desired, since the synthetic resin shrinks in the further manufacturing process. In this context, residues of the synthetic resin can remain in the cooling channels and block them.
It was already proposed that the encapsulation of the primary part be eliminated, in order to attain simple cooling. U.S. Pat. No. 5,751,077 describes a primary part of a linear motor in a sealed housing, whose coils are in direct contact with a coolant, since encapsulation is completely dispensed with. The housing of the primary part forms, together with the iron core, two flow chambers, between which the coolant flows past the coils and consequently provides for their cooling. However, due to the forces acting on the electric lines in such a linear motor, the failure to encapsulate and, thus, fix the coils, and in particular the electrical leads for the cells, in position does not produce a stable set-up.